This invention relates to a propulsion and suspension system for an inductive repulsion type, magnetically levitated vehicle, and more specifically, to a magnetically levitated ("maglev") vehicle which is propelled and suspended by a system which includes propulsion windings partially encircling the vehicle-borne suspension magnets.
Maglev development began more than two decades ago in the United States, Germany, Japan, Canada and England. In the United States, renewed interest has been directed toward magnetic levitation transportation systems in view of such factors as energy conservation, high speed transportation at ground level, economic and environmental problems associated with conventional systems, and competition from West Germany, and Japan.
The use of an electrodynamic suspension to provide levitation in maglev systems is well known in the prior art (see further, U.S. Pat. No. 3,470,828, issued Oct. 7, 1969, to Powell et al). A repulsive levitation (suspension) force is generated by the interaction between a rapidly changing magnetic field generated by superconducting magnets aboard the moving vehicle and eddy currents induced in the guideway. The guideway can be made of a continuous sheet of a non-magnetic conductor, such as aluminum, or of discrete coils, loops or slotted hollow tubular type structures of similar material.
The use of a linear synchronous motor (LSM) to propel a wheeled or levitated vehicle is also well known in the prior art (see further, Rhodes et al., "Magnetic Levitation for Rail Transport", 1981, pages 62-67), and several systems include a linear synchronous motor with electrodynamic repulsion system to provide propulsion by magnetic means (see further, U.S. Pat. No. 3,815,511, issued Jun. 11, 1974, to Dukowicz et al).
The Japanese MLU-002 electrodynamic suspension system is one of the most highly developed systems of this type in the world. Superconducting magnets on the vehicle react against conventional, normally conducting, coils in the guideway. In early tests, the superconducting magnets were placed in a horizontal position and reacted against horizontal coils on the bottom of the guideway. The superconducting magnets were later redesigned and located vertically, reacting with horizontal coils on the guideway for levitation and vertical coils located on the sidewalls of the guideway for guidance. The guidance coils are connected in a null-flux configuration to reduce the electromagnetic drag. Linear synchronous propulsion coils are also located on the sidewalls, but since they are symmetrically located with respect to the null-flux coils, they do not interact with them.
The basic components of the present magnetically levitated vehicle system have been identified and are well understood (see further, "Preliminary Design for a Maglev Development Facility", ANL/ESD-14). It is the object of this invention to devise a configuration of those components which will achieve the highest synergy, that is, which will minimize the negative effects and maximize the positive effects of one component on the others.
The present invention departs from the prior art principally in its configuration of the windings for the linear synchronous motor (LSM), in the use of multiple superconducting magnets to achieve better field profiles for propulsion, levitation, and guidance, and in its guideway configuration. In the prior art, the LSM coils were placed in the roadbed below or beside the superconducting magnets mounted on the vehicle. In some designs, these superconducting magnets were separated from the levitation magnets. The present invention uses a conductive guideway and LSM stator, both of which are shaped in the form of a slotted, tubular structure, to interact with a series of magnets on the vehicle to achieve levitation, guidance, and propulsion. The conductive guideway and the LSM are housed in the hollow, cylindrical core of the guideway structure. The guideway structure is part of the general support structure. A series of superconducting magnets located interior to the LSM and conducting guideway are connected to the vehicle by means of support members extending through the aligned, longitudinal slot of the LSM, conductive guideway, and the guideway. The longitudinal slot may be oriented in any preferred direction. The LSM includes a series of windings which are supplied with three phase AC electrical power to produce a traveling magnetic wave along the tubular axis of the guideway which interacts with the series of magnets attached to the vehicle to generate the propulsive force.
Since the self forces tend to expand a current carrying coil, it is easier to construct superconducting magnets having circular cross sections since the forces are constrained by the conductor coils.
The current in the series of superconducting magnets attached to the vehicle can be varied from magnet to magnet to alter the magnitude or polarity of the magnetic fields to achieve different objectives. If the same polarity is used in all of the magnets, the magnetic field will balloon out in the gaps between the magnets. Moving a series of magnets inside a stationary conducting tube and along the common axis, eddy currents will be induced in the conducting tube by the varying magnetic field. The magnetic field generated by these eddy currents will cause the magnets to be repelled from the tube, thereby centering the series of magnets along the axis of the tube. By adjusting the currents in each of the magnets, their lengths, their inside and outside diameters, as well as the spacing between the magnets, the profile of the magnetic fields experienced by the conducting tube can be altered to achieve the best repulsive force for levitation and guidance while experiencing the least electromagnetic power dissipation. Further, by altering these parameters, the axial and radial components of the magnetic fields outside the series of magnets can be varied. An axial force can be generated in this structure by encircling it with an array of slotted, hollow cylindrical coils to form three windings of different AC phases and energized by a three-phase power source to generate an axially directed traveling magnetic wave. When the objective is to provide the maximum propulsive force, the radial component of the magnetic field should be maximized. By energizing the magnets so alternate coils have opposite polarities, the field profile emphasizes the radial component of the field. Such structures can be used to produce either leviation and guidance forces or propulsion, levitation and guidance forces combined. Using these methods, the conductive guideway can be constructed with slotted, tubular sections for levitation and LSM coil sections for propulsion. These sections are alternated so that some of the magnets attached to the vehicle are in each section at all times. Generally, the propulsion coils would form a complete circle; however, since a slot is required for the support structures connecting the vehicle to the superconducting magnets, the LSM winding must allow for this. As indicated previously, the slot through which the vehicle is connected to the superconducting magnets can be located along any plane within the tubular structure.
The LSM windings are arranged so that alternating individual coils are wound in opposing directions using a continuous section of conductor; this allows for the formation of the slot in the completed solenoid coil. Since the windings are in opposition, the inductance is small, an asset when the winding is long since the reactive impedance and therefore the voltage required is minimized. The current between circular sections is primarily parallel to the magnetic field and results in little or no force. Since the magnetic fields alternate in polarity, the magnetic field decreases rapidly with distance from the magnets. The shielding currents induced in the guideway will also reduce the stray fields.
The slotted cylinder configuration of the present invention enhances the shielding from the magnetic field for the passengers and provides a protective enclosure for the guideway interior.
In addition, the invention provides for flexibility in adjusting the magnetic field profile resulting from the series of superconducting magnets to maximize propulsion or levitation or guidance.
In addition, the invention minimizes the reactive impedance, thus, lowering the power required and, therefore, the cost.
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.